The present invention relates to a method and device for the layered generation of thin volume grating stacks. The present invention further relates to a beam combiner for a holographic display.
Diffractive optical elements (DOE) play a special role in the manufacture of holographic direct-view displays. These elements, which are typically provided in the form of transparent films with presettable thickness, influence the light beams which strike them coming from an image-generating device, such as a large-area light modulator (SLM), through diffraction effects rather than through refraction. This way, the total thickness of the light-beam-influencing elements which are disposed downstream of the SLM in the optical path, seen in the direction of light propagation, and which preferably include light-refracting elements, such as lenses, prisms etc., can be kept very low in contrast to projection displays.
However, the influences which affect the light that is emitted by the pixels of an SLM which are necessary to generate a three-dimensional image sensation in the observer can only be realised by a multitude of film-type DOEs, which are disposed downstream of the SLM in the form of a large-area layer stack, for example. It is also desired for technological reasons in such arrangement that multiple stacked layers are generated in one continuous medium rather than subsequently joining individual films, e.g. by way of gluing. This is because in subsequently joined layer stacks there is the risk that the positions of the individual plane elements in the different layers in relation to each other may change, e.g. through shrinkage.
The structures which are used to influence the direction of the light which is emitted by the pixels of an SLM in a wavelength-specific manner through diffraction effects can be surface gratings or volume gratings. Volume gratings are typically understood to be three-dimensional grating structures which are recorded in a medium which is thick relative to the wavelength of the exposure light. Volume gratings have the advantage that multiple gratings can be generated in layers in a continuous medium, while surface gratings can only be disposed on one or either surface of a recording medium or recording material.
As is known from lithography, three-dimensional structures can be generated in a transparent and photosensitive recording medium, e.g. photoresist, by way of depth-specific focussing of the exposure light for which the recording medium is sensitised. Such a method is described in document US 2010/099051 A1, for example. This way, functionally different structures such as diffraction gratings can be generated step by step in different layers of the recording medium. However, the intensity of the illumination must here be controlled such that it only exceeds the sensitivity threshold of the recording medium in the particular layer.
Since DOEs are preferably diffraction gratings, it makes sense, however, to record the gratings in a single step through interference of two light waves which strike the recording medium at different angles. Such a method is described in document DE 197 04 740 B4, for example, in the context of the manufacture of a holographic display screen, where multiple volume gratings can be generated in different layers of a single recording medium, where said volume gratings can be assigned to light of different wavelengths, or where multiple volume gratings for light of different wavelengths are interleaved in one layer. One challenge is to ensure the capability of generating interference among the two light waves of the exposure light, which is here realised by way of reflection of the incident light beam.
The coherent superposition of a parallel pencil of rays which strikes a photosensitive recording medium at a certain angle and a pencil of rays which is generated at the exit surface of the recording medium by way of total internal reflection is taken advantage of for generating volume grating structures, as described in document U.S. Pat. No. 7,792,003 B2. The recording medium is here disposed next to the exit surface of a pivoted prism, where the angle at which the pencils of rays interfere is presettable through the rotation of the prism. This ensures a continuously variable control of the diffraction efficiency of the volume grating depending on the wavelength of the diffracted light. In addition, the structure of the generated volume grating can be influenced by rotating the prism during the recording such that the diffraction efficiency has the same high value for multiple wavelengths, so that it resembles a rectangular function. Other volume grating profiles can be generated through reflection of the incident exposure light wave at a profiled surface, which can in this case convert an incident parallel pencil of rays into a convergent or divergent pencil of rays, for example. However, one problem is to generate the volume gratings in a large-area recording medium, because the surface area of the recording medium is determined by the exit area of the prism.
When recording volume gratings, great importance is attached to the issue of depth-specific apodisation of these gratings, that is the longitudinal modulation or shape of the refractive index profile in the Z dimension (i.e. perpendicular to the surface of the recording medium, for example, or, more general, along the direction of propagation of the wave field which is used for the reconstruction), in addition to the modulation in the X and/or Y dimension, i.e. parallel to the surface of the recording medium, for example. This method allows volume gratings to be recorded which when diffracting light waves specifically suppress side peaks of the diffracted order, for example, and which, more generally, show controllable angular and wavelength selectivity. Such a method is described in the publication “Coupled-wave analysis of apodized volume gratings” by J. M. Tsui et al., Optics Express, Vol. 12, No. 26, pp. 6642, where in a recording medium made of photosensitive glass the refractive index modulation which is available for subsequent coherent illumination is depth-specifically reduced by way of preliminary exposure with incoherent light of reduced penetration depth from either of the two outer surfaces such that the envelope of the refractive index modulation in the Z dimension shows a Gaussian profile, for example, in very rough estimation during the coherent illumination.
A thus recorded volume grating is then superposed by this absorption profile, which is for example generated by short-wave UV radiation, as an apodisation function in the Z dimension. However, this absorption approach does not allow multiple thin volume grating layers to be given respective longitudinal apodisation profiles in a thick recording medium. This method, which illustrates the prior art, only allows apodisation functions to be generated which have been generated by single- or double-sided absorption profiles which are proportional to I0×e−αz, where α is the absorption coefficient of the wavelength used for incoherent preliminary exposure, which is a short-wave UV radiation, for example. Exponentially decreasing functions only are thus availably as apodisation functions for the refractive index modulation. Consequently, the range of generatable apodisation profiles is limited to few functions, which are continuously decreasing from the outer surface of the recording medium inwards. Moreover, different wavelengths must be provided with different absorption coefficients to be able to generate anything more than a simple apodisation profile by way of preliminary exposure.